HOW TO OBSERVE. 
65 
Again : a person who first begins to observe geo- 
logical phenomena is very apt to mistake an under 
for an upper stratum. For example, suppose (in 
figure 20) a hill to be covered with vegetable soil, and 
Fig. 20. 
a quarry or pit to be made in it near the bottom, as 
at «, and the stone was discovered to be sandstone ; 
if another pit was sunk higher up at which cut 
into limestone, it might be supposed, because the 
limestone is met with at a higher level, that it lies 
over the sandstone stratum, when it is, in reality, 
below it. And so at c, at the top of the hill, he 
might suppose that the state which crops out on the 
summit was above both the others, when, in fact, it 
is the lowest of the three in the order of position, 
though the highest in point of elevation. 
We often read in geological works of valleys of 
denudation ; these may be explained by the follow- 
ing figure. 
Fig. 21. 
Here we r^erceive that several tracts of the upper 
strata are wanting between a and b (fig. 21), and that 
the lower strata have been laid bare either by the 
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